The present invention relates to a roll cooling device and method for cooling work rolls of a rolling mill, and more particularly to a roll cooling device and method for cooling work rolls of a strip rolling mill for rolling a sheet material or strip steel.
During a rolling operation, rolls of a rolling mill are continuously heated by a work heat due to the plastic deformation of the rolled material, a frictional heat generated between the rolled material and the rolls and the like. In particular, in case of hot rolling, since the rolled material is kept at a high temperature of about 1200.degree. C., the resultant temperature of the rolled material becomes much higher. Also, the rolls are further heated by heat generation due to slippage between the rolls and the rolled material.
The heating of the roll first starts from the roll surface which is brought into contact with the material and subsequently, the heat is conducted from the surface radially inwardly toward the center of the roll. Also, with respect to a longitudinal direction of the roll, the heat is conducted from the longitudinal central portion of the roll toward both ends of the roll. As a result, with respect to the longitudinal direction of the roll, the temperature of the central portion is kept highest and the temperature is gradually decreased toward the ends of the roll. Consequently, due to the heat expansion, the diameter of the roll becomes larger in the central portion than at both ends. In case of hot rolling, this difference in diameter due to heat expansion reaches about 0.1 to 0.4 mm.
When the roll has a larger diameter in its central portion and a smaller diameter at both end portions, a thickness of the central portion of the rolled material is smaller than that of the edge portions of the rolled material, thus causing a problem that the rolling precision deteriorates. Also, when the temperature of the roll is elevated, the roll is thermally stuck to the material, resulting in degradation in quality of the product.
Accordingly, the rolls of the rolling mill must be always cooled during the rolling operation. For this reason, the cooling of the rolls has been performed by injecting cooling water onto the roll surface apart from the position of the rolls by means of spray nozzles. An average heat transfer coefficient from the roll surface is limited to 3,000 kcal/m.sup.2 hr .degree.C. and hence, the cooling performance is limited. To obviate such defects, various attempts have been made to increase a flow rate of the cooling water or to increase a pressure of the cooling water. However, this is also limited and is not sufficient to cool the roll.
To solve such problems of insufficient cooling, Japanese Patent Examined Publication No. 12322/1980 proposes an improved method. According to this prior art method, a cooling water guide having a shape in corformity with an outer periphery of a roll is arranged in constant spaced relation with the roll, and the cooling water is forcibly supplied into a clearance between the cooling water guide and the roll so that an average heat transfer coefficient is increased to about four times of the conventional one. However, such a prior art method suffers from the following disadvantages.
(1) The roll is worn by rolling the material and is periodically abraded by 0.1 to 0.5 mm in terms of the diameter. In the cooling apparatus as disclosed in the above-mentioned Japanese publication No. 12322/1980, the cooling water guide is made of flexible material and is deformed by a fluid resistance of the cooling water supplied between the cooling water guide and the roll so that the cooling water guide may follow the change in diameter of the roll. However, according to such a method that the cooling water guide is deformed by utilizing the fluid pressure, it is impossible to obtain an increased fluid pressure and it is difficult to increase the deformity. The resultant deformity is only enough to follow the roll diameter change of about several millimeters. On the other hand, the extent of roll abrasion from new use finally reaches about 10% of the roll diameter. The deformity of the cooling water guide plate cannot meet such a roll diameter change and it is impossible to sufficiently cool the roll. Also, if the flow rate of the cooling water would be decreased for some reason, there is a fear that the fluid pressure would be reduced so that the cooling water guide plate and the roll would be in contact with each other. This causes a problem of the cooling water guide plate being undesirably stuck to the roll. PA0 (2) Since the cooling water guide is located extremely near to the roll, it is necessary, upon replacement of the rolls, to move the cooling water guide away from the roll. This makes the structure complicated and increases the time needed for the roll replacement.
In view of an economical point, it is preferable that the clearance or gap between the cooling water guide plate and the roll be reduced as much as possible, to reduce the amount of the cooling water passing through the clearance (cooling water passage). In order to maintain this extremely small clearance accurately, pipings and tubes must be flexible because there is a necessity to move the cooling water guide as described above. However, generally used rubber hoses impose a local load to the cooling water guide due to their rigidity and hence, it is difficult to keep the clearance constant.
Also, Japanese Patent Unexamined Publication No. 83658/1979 shows a roll cooling device in which cooling headers are internally formed and roll cooling pads provided with a plurality of cooling water injection ports communicating with the cooling headers are supported by bearing boxes. However, in the conventional roll cooling device, since the cooling water supply to the cooling pads is carried out by using water supply holes passing through the bearing boxes, a mechanical strength of the bearing boxes is reduced, and in view of the roll replacement, it is necessary to provide a means for attaching/detaching external water supply tubes connected to the water supply holes formed in the bearing boxes. This makes the structure complicated and increases the time needed for the roll replacement.